1. Field of the Invention
This invention relates generally to a regenerative laser amplifier system and its methods of use, and more particularly to a low-gain regenerative amplifier system with a simplified configuration that eliminates the leakage of the seed laser.
2. Description of Related Art
Regenerative amplifiers are typically used to increase the energy of an optical pulse. These pulses can be of any duration but a particularly useful range includes ultrafast pulses with picosecond or femtosecond duration. The laser oscillators that produce these ultrafast pulses are often limited in the energy per pulse that they can produce with a typical energy being 1 to 100 nanojoules. A regenerative amplifier for these ultrafast pulses can increase the energy to the millijoule range, a gain of 106.
A regenerative amplifier contains a gain medium surrounded by a resonant cavity. Within this cavity are means for switching the pulse in and out of the cavity. This is distinct from a multi-pass amplifier where there is no cavity or need to switch the pulse. In the case of the multi-pass amplifier, the pulse makes several trips at slightly different angles through the gain media. One advantage of the regenerative amplifier is that a larger number of passes through the gain media can be used, and thus a larger gain can be obtained.
A regenerative amplifier system contains not just the amplifier, the cavity and the means for switching the pulse, but also the oscillator and a means for isolating the oscillator from feedback. The oscillator produces a continuous train of pulses with a typical repetition rate of 80–100 MHz. The switching means is typically an electro-optic device but can also be an acousto-optics device. The most commonly used electo-optic device is a Pockels cell where a voltage is applied to a crystal and the polarization of the pulse passing through the cell is altered for example from horizontal polarization to vertical polarization. A polarizer is an optical device that passes horizontal polarization and reflects vertically polarized pulses. The Pockels cell, when used in conjunction with a quarter wave plate and the polarizer provides the means for switching the pulse in to or out of the cavity. For example, once the pulse has been trapped in the cavity and been amplified to the point where the gain has saturated and no further amplification is possible, a voltage would be removed from the Pockels cell and the polarization of the pulse would be rotated from horizontal to vertical during the next pass of the pulse through the Pockels cell and waveplate. When the pulse is then incident on the polarizer, it will be reflected from the cavity. This then constitutes the output beam of the regenerative amplifier.
There are several options for switching the pulse in to and out of the cavity. Two Pockels cells may be used as described in U.S. Pat. No. 5,790,303. One Pockels cell is used to switch the pulse in and the second is used to switch the pulse out. The pulse exits the regenerative amplifier from an intracavity polarizer. The pulse can be injected in the amplifier by another polarizer or off the Brewster surface of the gain medium, as shown in FIG. 4 of the patent. During the time before the seed pulse is switched into the cavity of the regenerative amplifier, the pulses from the oscillator enter the cavity, pass through the gain medium and are reflected back towards the oscillator itself. This poses a severe problem since ultrafast oscillators are very sensitive to feedback. Reflection of as little as 1% of the light output from the oscillator back into the oscillator can cause instability and large fluctuations. A Faraday isolator is required to block the train of seed pulses from the returning in the direction of the oscillator. Thus a typical regenerative amplifier system consists of two Pockels cells and one or two polarizers in the regenerative amplifier cavity and a Faraday isolator located between the oscillator and the regenerative amplifier.
Some gain media do not produce as much gain per pass as others. These low-gain gain media are usually chosen for the wavelength they operate at or the ability to produce shorter pulses. For low-gain regenerative amplifiers, it is imperative to reduce the losses per pass in the cavity. Thus most low-gain regenerative amplifiers use only a single Pockels cell and a single polarizer within the cavity. Thus the same polarizer and Pockels cell are used to switch the pulse in to and out of the cavity. This requires more sophisticated high voltage electronics to drive the Pockels cell. The problems of feedback of the seed pulse to the oscillator still exists and a Faraday isolator is almost always used to prevent this feedback. Occasionally, prior art systems have used a 4% reflector instead of the Faraday isolator, however this does a poor job of isolation. A new problem occurs however, when a single Pockels cell and polarizer are used. The seed pulses that are incident on the regenerative amplifier cavity prior to one pulse being switched in to the cavity are now rejected by the Faraday isolator and thus do not return to the oscillator. Instead, these seed pulses are directed to the output beam of the regenerative amplifier. Thus the output of the amplifier consists of a large number of undesired seed pulses in addition to the amplified pulse. An example of such a system is given in “Diode-pumped Nd:glass kilohertz regenerative amplifier for subpicosecond microjoule level pulses”, by A. Braun, X. Liu, G. Mourou, D. Kopf and U. Keller, in Applied Optics vol. 36, no. 18 page 4163–4167. The authors state, “This background power level was 3 mW.” They conclude that “both the background power level and postpulses could be eliminated with a suitable electro-optic pulse selector” although they do not demonstrate how this may be accomplished.
One other configuration is described in “8-TW 90-fs Cr:LiSAF laser”, by P. Beaud, M. Richardson, E. J. Miesak and B. H. T. Chai, in Optics Letters vol. 18, no. 18, page 1550–1553. The authors use a single Pockels cell before the cavity to pick a single pulse for injection and a single Pockels cell and polarizer in the cavity to switch the pulse in. They do not switch the pulse out however and instead let a train of output pulses emerge from the end mirror of the cavity. Two additional Pockels are then required after the cavity to choose a single amplified pulse from this pulse train.
There is a need for a low-gain regenerative amplifier with a minimum of intra-cavity elements. There is a further need for a low-gain regenerative amplifier without leakage of the seed pulses as part of the output beam. There is yet a further need for a low-gain regenerative amplifier with a minimum number of expensive components. There is yet a further need for a low-gain regenerative amplifier with a high contrast ratio.